a) Field of the Invention
The invention relates to a system for delivering pressurized liquid to a flow cytometer, and in particular to a system having an improved liquid accumulator/pump control means for providing continuous modulation of power to a liquid pump drive means.
b) Description of the Related Art
Flow cytometry apparatus has commonly used a liquid suspension of particles ensheathed by a particle-free liquid wherein this coaxial flow passed through an analysis region and thence often to a particle sorting means. Such coaxial flow systems are shown in an article by P. J. Crossland-Taylor, Nature 171,37 (1953) and in U.S. Pat. No. 3,826,364, which are hereby referred to and incorporated herein. Sheath liquid is usually a phosphate buffered saline solution and is usually supplied to the analysis region from a closed reservoir pressurized by air from an air pressure regulator connected to a source of air at a higher pressure (note items 16, 26 and 22 of U.S. Pat. No. 3,826,364). Since particle analyzers and particle sorters often depend on consistent liquid flow velocities through the analysis region, this air pressurized sheath supply system has the following shortcomings:
1) As the sheath supply empties during operation of the flow cytometer, the loss of head causes a decrease in liquid flow; PA1 2) Changes in sheath liquid temperature cause changes in sheath liquid flow rate due to changes in liquid viscosity. Changes in liquid temperature can result from a) changing ambient air temperature at the sheath reservoir, b) reservoir replenishment with sheath liquid at a different temperature such as from a remote storage area and/or steam sterilization of sheath liquid; PA1 3) Replenishment of sheath liquid is inconvenient, requiring stopping operation of the flow cytometer, de-pressurizing the reservoir, opening and refilling the reservoir, re-pressurizing the reservoir and restarting the flow cytometer; PA1 4) The pressurized reservoir has often been a stainless steel ASME pressure vessel which is both expensive and unsuitable for visual observation of liquid level in the reservoir; PA1 5) Air dissolves in the sheath liquid in time and can later be released as bubbles as the liquid loses pressure while flowing through filters, valves, and conduits to the analysis and sorting regions. Bubbles in these regions often prevent proper analysis or sorting functions; and PA1 6) When a pressurized air supply is not available at a flow cytometer installation, then a separate air compressor, motor, reservoir, and controls must be provided. PA1 a) to provide essentially pulse-free constant pressure sheath liquid to a flow cytometer unaffected by liquid level changes in the supply reservoir; PA1 b) to employ an unpressurized supply reservoir which is easy to refill, is simple and low cost, may be raised and lowered without affecting the sheath supply pressure, does not introduce air into the sheath liquid, does not require a separate air supply and valves, may be sized large to reduce replenishment frequency, may be transparent for visual observation of liquid level, and may be replenished without stopping operation of the flow cytometer; PA1 c) to provide for manual or automatic adjustment of the sheath liquid pressure as required to compensate for variations in sheath liquid temperature and thereby maintain sheath flow rate substantially constant and thus maintain critical flow cytometer timing such as: PA1 d) to provide a novel liquid accumulator which can accept the liquid volume delivered by one stroke of a diaphragm pump with negligible change (less than 0.1%) in liquid pressure; and PA1 e) to provide a self-priming pump in a sheath liquid supply system where sheath liquid contacts only non-metal parts thus avoiding metal corrosion and contamination of the sheath liquid.
Attempts to use gear or centrifugal pumps to pressurize sheath liquid, usually a phosphate buffered saline, have not produced practical designs. Neither pump is inherently self-priming so initial start up or restart after running out of liquid requires the operator to perform special procedures such as bleeding air from the system. If either pump is kept running when liquid flow through the cytometer stops, then the pump will tend to overheat and be damaged. Solutions such as an overflow/over pressure line for returning pressurized liquid back to the supply reservoir or stopping the pump add cost and complexity. Also gear and centrifugal pumps suitable for long life operating with corrosive saline are expensive.
Many of these shortcomings of gear or centrifugal pumps are avoided by diaphragm pumps, particularly those with polymer housings and with elastomer diaphragms and check valves. However, diaphragm pumps require a liquid accumulator to supply pressurized liquid during the refilling stroke of the pump. Common accumulators employ a piston loaded by a spring or a bladder loaded by compressed gas or combinations thereof (as is shown in U.S. Pat. No. 4,278,403. This patent shows an accumulator 35 which operates a pump P via a switch 43 in an on/off mode from a pressure movable partition element, piston 36. This on/off mode of pump control with its dead band between On and Off conditions results in significant changes in pressure in accumulator 35. In addition the friction from seals for piston 36 and stem 41 produce inaccuracies in the sensing of pressure in accumulator 35. Also these seals are subject to wear and leakage which limit the durability of accumulator 35.